High voltage power supplies have typically been low frequency networks characterized principally by their mass, bulk and relative inefficiency. Recent advances in such fields as diagnostic x-ray devices (e.g., the Low Intensity X-ray and Gamma-ray Imaging Device disclosed in U.S. Pat. No. 4,142,101) have established a demand for portable, battery driven power supplies able to deliver a plurality of regulated, static output voltages on the order of tens of kilowatts for periods of several hours. The operational characteristics of such devices usually require that each potential furnished by a power supply be separately adjustable.
Currently available battery operated power supplies typically have a push-pull inverter stage coupled across a center tapped primary winding of a saturable core step-up transformer. When the core saturates, a potential is induced in a second winding which causes the inverter stage to reverse its mode of conduction. The frequency of operation of the inverter stage is principally controlled by the saturation time of the transformer. Often in such configurations, a feed-back potential is derived from a secondary winding and applied to control the amplitude of the voltage applied by the inverter stage to the transformer. A voltage multiplier stage driven by another secondary winding is used in such configurations to provide an increased output voltage while a voltage divider stage converts the output from the multiplier stage into a plurality of different output signals.
The use of voltage induced in the secondary winding, as practiced by these prior art power supplies for switching the conduction mode of the inverter stage causes the output current provided by the transformer's secondary winding to be generated as a series of square waves pulsing at the same frequency at which the inverter is being switched. In effect, such power supplies rely upon the transformer to establish the switching frequency of the inverter stage; harmonics of the switching frequency are, therefore, included in any signal appearing across the secondary windings of the transformer. The presence of such harmonics introduces substantial undesired ripple into the output signals provided by the power supply.
Moreover, the use of a divider stage to provide multiple output signals prevents independent adjustment of the output potentials. Furthermore, the presence of high current spikes occurring during changes in core saturation and the sudden change in the amplitude of current which occurs at each transition between pulses causes a reflected ripple current which, in turn, causes electromagnetic noise that detrimentally interferes with the operation of any neighboring electrical equipment. Also, the saturable core of the transformer is a significant source of energy loss, a factor which renders these configurations unsuitable for use in battery powered, high potential supply sources. The presence of such ripple renders this type of power supply unsuitable for use in applications where both a constant high voltage and a well regulated but much lower amplitude voltage floating at the high voltage are required as output signals because the ripple from the high voltage stage destroys the regulation of the low voltage stage.
Attempts to improve the regulation of output potentials have included efforts to compensate for variations in output voltages due to causes such as changes in loading. Such efforts typically rely upon a pulse width modulator to regulate a chopping transistor driving the center tap of the primary winding. In these configurations a feedback loop, such as a current sensing stage is often used to provide an analog signal for controlling the duty cycle of the modulator in proportion to changes in the loading of the transformer's secondary winding. This type of power supply is not suitable for providing high voltages, however, because of a lack of electrical insulation between the input and output sides of the circuit. Moreover, such power supplies require synchronization between the pulse width modulator and the transformer, a feature which restricts the range over which the duty cycle of the modulator may be varied to compensate for changes in loading of the power supply.
Other power supplies have attempted to obtain well regulated output signals by using a separate control circuit having ancillary oscillator and base drive stages to regulate switching of transistors driving a transformer in a power converting stage. The ancillary circuits themselves require a power supply. The presence of such ancillary circuits and their individual power supply undesirably adds to the complexity and physical bulk of the overall design. Moreover, the control circuit in such power supplies is driven by a feedback signal obtained directly from the output terminals of the power supply, a feature which prevents insulation of the control circuit from the output voltages and, therefore, renders these power supplies unsuitable for generation of high output voltages.
Recent efforts to enable a power supply to provide high voltages suitable for operation of x-ray tubes have included a variety of capacitive discharge circuits. One power supply, for example, included a motor driven, rotating commutator providing sequential discharge of individual capacitors through the primary winding of a step-up transformer. Although capacitive discharge type power supplies are adequate for providing high voltage impulses of short duration, without extensive, power consuming filtering, the transient phenomenon accompanying discharge renders such power supplies unsuitable for providing well regulated output voltages. Additionally, the presence of motor driven, rotating communators makes such power supplies less than ideal for use in small, portable devices.